The present invention relates generally to a novel method of seismic exploration, and more particularly, to a method of shear wave seismic exploration for identifying gaseous hydrocarbon-containing formations and for inferring changes in the geological character of the formations. Shear wave seismic data, including a plurality of shear wave seismic signals or traces representative of the earth's response to imparted seismic energy, are obtained by seismic receivers. A measure of the shear wave reflection coefficient is obtained for selected seismic events within the seismic data. Attributes quantitatively descriptive of variations in the shear wave reflection coefficient as a function of incident angle are developed from shear wave seismic signal amplitudes for the selected seismic events. Predetermined combinations of such attributes can provide the geophysicist with measures of the rigidity contrast and density contrast for selected seismic events which can be extremely accurate diagnostic tools for identifying and quantifying gaseous hydrocarbon-containing formations and for inferring changes in the subterranean formations.
In the continuing search for hydrocarbons contained in the earth's subterranean formations, exploration geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subterranean formations, recording the reflected seismic waves and processing the recorded seismic data to produce seismic signals or traces. Such seismic signals or traces contain a multiplicity of information, e.g., frequency, amplitude, phase, etc., which have been related to formation structure, lithology or pore fluid content. More recently geophysicists' interests have focused on variations in the seismic signal amplitude and compressional wave reflection coefficient as a function of range or incident angle. Exemplary of such focus are: Ostrander, U.S. Pat. No. 4,316,267 and 4,316,268 and Wiggins, et al., U.S. Pat. No. 4,534.019.
In particular, Ostrander indicates that progressive change in compressional wave reflection coefficient as a function of angle of incidence can occur, and more likely than not, such progressive change is an indicator of the presence of a gas-bearing formation. Specifically, after specified processing, progressive seismic signal amplitude change, in an increasing or decreasing manner with increasing range, is the criterion for identifying gas-bearing formations. Ostrander also discloses a method for seismic signal enhancement to improve the visual resolution of such progressive changes in compressional seismic wave signal amplitude as a function of range or incident angle.
On the other hand, Wiggins indicates that for a common depth point gather of reflected compressional wave seismic energy, coefficients descriptive of the variation of such seismic trace signal amplitudes as a function of sin.sup.2 .theta. can be summed to obtain a measure of the shear wave velocity reflectivity of the earth's subterranean formation
However, the techniques of Ostrander and Wiggins employed to locate gaseous hydrocarbon-containing formations and to identify subterranean features, have been limited to the evaluation of the compressional wave reflection coefficient as observed in compressional wave seismic energy and in the variations of the recorded compressional wave seismic signal or trace amplitude as a function of incident angle. In fact, the focus of those skilled in the art has been limited to compressional wave seismic data, apparently due to the conventional belief that reflected shear wave seismic energy is insensitive to changes in the pore fluid content of the earth's subterranean formations, whereas the reflected compressional wave seismic energy is sensitive to changes in pore fluid content of the earth's subterranean formations.